Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor (“directly driven”) either directly or through the use of a gearbox.
Wind turbines generally comprise a tower upon which a nacelle is mounted. A rotor comprising a hub and a plurality of blades may be rotatably mounted in said nacelle. When erecting a wind turbine, generally the tower is constructed first. Then the nacelle may be mounted on top of the wind turbine and, subsequently, the hub and blades may be hoisted towards the top of the tower and mounted in the nacelle. It is known to hoist a complete rotor assembly (a hub comprising all blades) and installing it on the nacelle. However, it is also known to hoist an incomplete rotor (only a hub, or a hub comprising a selection of the blades) and installing it. Subsequently, the remaining blades can be hoisted one by one from a ground level towards the rotor hub. Alternatively, it is known to hoist the nacelle together with the hub, and subsequently hoist the blades, one after another.
A clear trend in the field of wind turbines has been to increase the size of the turbines. With the goal of converting more energy and producing more electricity, towers have become higher and blades have become longer. In order to hoist a blade from a ground level towards the hub it is known to use two large cranes. For example, the blade may be horizontally hoisted by holding it at two regions thereof using two cranes. However, the use of such large cranes can be very expensive.
A further trend in the field of wind turbines is to place them offshore. The use of boats carrying large cranes may be even more expensive than using cranes on land.
It is known to avoid using more than one crane to hoist a blade up to a hub or down therefrom by providing pulleys on the hub. US2010139062 discloses a lifting arrangement for vertically lowering and raising a single rotor blade of a wind turbine. Lifting fixtures are symmetrically installed on a hub surface about the rotor blade being replaced. Lifting wires from coordinated ground winches pass over the lifting fixtures to support the blade and attach to a blade harness. The blade harness attaches over a substantial length of the rotor blade, distributing the blade weight broadly. A tail pick crane facilitates transfer of the rotor blade between a vertical and a horizontal position. But with this arrangement the blade has to be handled by means of a harness, which requires great care in order not to damage the blade, because the maximum pressure that can be applied to the blade surface without spoiling it is rather low.